Free-volume elements in soft organic materials have been focused upon to improve membrane separation performance in chemical products as well as for energy conversion and storage applications [P. M. Budd, N. B. McKeown, D. Fritsch, Polymers with cavities tuned for fast selective transport of small molecules and ions, J. Mater. Chem. 2005, 15, 1977; W. J. Koros, Fleming G. K., Membrane-based gas separation, J. Membr. Sci. 1993, 83, 1; S. A. Stern, Polymers for gas separations: The next decade, J. Membr. Sci. 1994, 94, 1].
The free volume element size and distribution play a key role in determining permeability and separation characteristics of polymers. Among typical polymeric membranes, glassy polymers have exhibited good gas separation performance with high selectivity, however, permeability of glassy polymers is poorly suited to practical applications [M. Langsam, “Polyimide for gas separation, in Polyimides: fundamentals and applications”, Marcel Dekker, New York, 1996; B. D. Freeman, Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 1999, 32, 375].
Even though some glassy polymers with ultra-high free volume such as poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(4-methyl-2-pentyne) (PMP), and copolymers of 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxide and tetrafluoroethylene (amorphous Teflons AF) exhibited extremely high gas permeability, they still had very low performance in selectivities. [K. Nagai, T. Masuda, T. Nakagawa, B. D. Freeman, I. Pinnau, Poly[1-(trimethylsilyl)-1-propyne] and related polymers: Synthesis, properties and functions, Prog. Polym. Sci. 2001, 26, 721; A. Morisato, I. Pinnau, Synthesis and gas permeation properties of poly(4-methyl-2-pentyne), J. Membr. Sci. 1996, 121, 243; A. M. Polyakov, L. E. Starannikova, Y. P. Yampolskii, Amorphous Teflons AF as organophilic pervaporation materials: Transport of individual components, J. Membr. Sci. 2003, 216, 241].
A great deal of research has endeavored to produce ideal structures having precise cavities for high gas permeability and high gas selectivity. As a result of this research, there has been remarkable development of polymer membranes exhibiting high gas-separation performance. For example, designs for nanocomposites, hybrid materials and complex polymers were considered to impart large free volume to polymers.
Of these, methods to realize intermediate and small cavity size distributions were reported recently [H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas, S. T. Mudie, E. Van Wagner, B. D. Freeman, D. J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 2007, 318, 254. 38].
Lee et al. suggested that completely aromatic, insoluble, infusible polybenzoxazole (TR-α-PBO) membranes can be prepared by thermally modifying ortho-hydroxyl group-containing polyimide aromatic polymers through thermal rearrangement to molecular rearrangement at 350 to 450° C. [H. B. Park, C. H. Jung, Y. M. Lee, A. J. Hill, S. J. Pas, S. T. Mudie, E. Van Wagner, B. D. Freeman, D. J. Cookson, Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 2007, 318, 254. 38].
TR-α-PBO membranes have advantages of excellent gas separation performance and superior chemical stability and mechanical properties, surpassing the limitations of typical polymeric membranes (i.e., the Robeson's upper bound). [L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 1991, 62, 165, L. M. Robeson, The upper bound revisited, J. Membr. Sci., 2008, 320, 390]. However, in spite of extremely high permeability in CO2 separation, TR-α-PBO still exhibits low selectivity for small gases such as hydrogen and helium.